1. Field
Embodiments of the present invention relate, for example, to an overheat protection apparatus, and particularly to an overheat protection apparatus which limits current to a motor in accordance with a motor temperature estimated from a heat generation amount by energization current to protect the motor and motor peripheral apparatus from overheating.
2. Description of the Related Art
Electric power steering systems, wherein pivotal assisting force is applied from an electric motor to a steering shaft to reduce the steering force, are sometimes provided in an apparatus that includes a steering shaft that is rotated by a bar handle, a steering wheel or the like to steer a vehicle.
Japanese Patent Laid-Open No. 2008-54440 discloses a controlling apparatus for an electric power steering apparatus which estimates a winding temperature of an electric motor and carries out temperature protection control of the motor based on the estimated temperature.
Generally, where the winding temperature of a motor is estimated, a value I of current flowing through the winding and a resistance value R of the winding are utilized in accordance with the Joule's law. In particular, the heat generation amount Q accumulated within energization time t is calculated in accordance with the following:Q=I2×R×t  (expression 1).
Although the heat generation amount Q with which a temperature can be estimated is calculated in accordance with this expression 1, in order to estimate the temperature with a higher degree of accuracy, also the heat radiation amount can be taken into consideration. The expression 2 given below is an estimation expression for calculating a cumulative value T of the heat mount including a constant a as a heat radiation amount correction term.Cumulative value T=Σ(K'I2−a)  (expression 2).
According to this expression 2, the heat generation amount when the power steering apparatus operates to energize the electric motor is integrated for the energization time period to estimate the temperature, and the constant a is subtracted as a heat radiation amount. The constant a in the expression 2 is set to a very low value so that, in order to estimate the temperature to a rather high value to ensure the temperature protection, the cumulative value T may return to zero in a period of time longer than the time period in which the temperature returns from the highest temperature to the room temperature when the energization is stopped at the highest temperature of the winding. This is because, if the constant a is excessively high, then the inclination that the cumulative value T becomes low becomes noticeable and the winding temperature is likely to be estimated lower. Where the energization is not carried out for a long period of time, the cumulative value T returns to zero with the constant a. It is to be noted that, in the expression 2, the coefficient K is an integration coefficient and is determined in advance by an experiment so that the calculated value may approach an actually measured value.